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Gabriel Wilson

Exposing for Low Light - LOG C

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I have been tasked to shoot a very moody film on Alexa mini. I am familiar with the camera but want to get the most out of the camera in low light/night scenes. I am curious how people expose for low light night and if I need to change any camera settings during the night scenes. The film has a lot of contrast and I want to make sure I don' crush the blacks too much .

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Im not sure Log is a good choice for low light in general.. not much data allocated down there .. well this is true of Sony Slog3 ,which is pretty much the same / a copy of Arri Log C ..  could be wrong .. ?

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Well, if you're worried about noise, you can pick a lower ISO, but I wouldn't shoot anything but Log-C or Arriraw on an Alexa, the whole point is to record all that dynamic range, and even a low-light scene might have a bright lamp in the frame somewhere.

If you record Log-C or raw, the black level, the contrast, etc. -- besides lighting contrast ratios -- is whatever display gamma you pick for monitoring and dailies -- standard Rec.709 or your own LUT.

Not sure if you want high-contrast lighting but a milkier, softer contrast display LUT or you want lower-contrast lighting and deeper blacks from the display LUT.  Generally the second gives you more options in color-correction since you are recording more shadow detail and then adding contrast to lose some of it, rather than recording very little shadow detail and being stuck with that, or getting milky, noisy blacks trying to lift detail up.

Unless I have to balance the lighting with a real candle or some other low-level source that I cannot make brighter, then for dark scenes, I lower the ISO from 800 to 500 to get better noise, more shadow detail. 

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Interesting. I usually monitor rec.709 on the mini. Any other build it LUTs not sure I will have time to build one before the shoot.

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Ideally want deeper blacks from the display. In rec.709 does it make sense to over expose the blacks on the display LUT? Can false colors help expose blakc level?

 

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ARRI Rec.709 is the display LUT if you select that in the Alexa menu.  You'd have to create your own version of Rec.709, a new LUT, but with the blacks crushed a little more.

I don't know what you mean by "overexpose the blacks" -- you can't expose the blacks separately from the rest of the image. If you mean give more exposure overall, that's the same thing as selecting a lower ISO.  Then the noise will be lower and you'll record more shadow detail at the expense of some highlight detail (the Alexa records the same range at all ISOs, but only at 800 ISO is the number of stops below and above middle grey the same.)

If you mean "add more fill light and then crush the blacks to compensate" you'd have to create your own LUT to do that (or just crush the blacks on every monitor on set and in the editing room!)
False colors can tell you where your shadows are falling but you can't expose them independently of the overall image.

"Black" is a separate issue than shadow detail or lack of it -- I would describe things in terms of shadow detail and contrast. Black is just a value and most of the time, we want it to be "0" in the final color-correction. You can crush detail below "0" if you want or lift it above "0" (at the risk of milkiness if there is no detail to be lifted, or that detail is buried in the noise floor.)

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Basically you:

1) Pick an ISO level that gives you the noise level you want to work with and gives you the balance between highlight detail compared to shadow detail you like.  The higher the ISO, the higher the noise and the more overexposure detail recorded and the less shadow detail.  The lower the ISO, the lower the noise and the more shadow detail recorded and the less overexposure detail recorded.

2) Pick a display gamma LUT or create your own for monitors and for dailies. The ARRI Rec.709 choice in the camera is OK, the blacks are black, the saturation is good.  Sometimes I find the highlights look a little yellowish to me.  ARRI has a Look generator software that you can use to build a Look file but it's better to create a LUT at the post house using test footage that then can be applied on set and to dailies.

3) Use decent monitors and something like a waveform or false color display as well.

4) LIGHT the scene for the look you want.

Then everything should be fine unless you change your mind in post.

I will give one precaution.  If you are shooting in very dark sets and viewing in a black room, the monitor is the brightest light in the space so staring at it, everything on screen looks brighter than it would in a room with some ambient lighting.  So if you are exposing by monitor, there is a tendency to underexpose to make it feel dark on the monitor, but when viewed later, everything looks too dark.  So either fight that tendency, mentally compensate for it, or if you feel the urge to close down the iris a little more to make things darker overall, try lowering the ISO instead to compensate -- the image will get darker and get recorded darker, but the amount of light hitting the sensor is the same and the noise is lower on the recording.  So let's say you drop from 640 ISO to 500 ISO (one-third stop) to darken the image by one-third of a stop, but in post you end up brightening it by one-third stop. You'd just be going back from 500 ISO to 640 ISO.

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Apologies, I meant add fill and crush. But your suggesting to not do that If I am using the built in rec.709? In reference to the shadow area is there a suggested IRE when you are trying to avoid noise or is it more a matter of taste? 

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Trying to be more clear I guess I am asking when one shoots a night scene where do the shadows usually fall so you don't crush the blacks but it still looks natural.

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The black level is set by the display LUT so you can't "crush the blacks" on set other than by changing the LUT.  So if you shoot the whole movie with one LUT, then there is one black level (unless you lift it by fogging the image somehow, like in smoked set or from a flare in the lens) and one display contrast, one saturation level, etc.  So if you like the black level of the preset ARRI Rec.709 LUT inside the Alexa, then use that. It's not going to change just because you're shooting at night. 

Crushing the blacks is something you have to actively do with the color-correction or the LUT, so were you planning on color-correcting on set? Creating new LUT's on the set?  It's possible but I don't think that's what you are asking.

You pick a LUT for it's black level and contrast / saturation, etc. You light for that LUT.  So pick a LUT and light the scene so the shadows look natural.

The main problem people run into at night is that they try to lift the blacks and shadows because they can't light for what they want, they are trying to use some natural ambience out there.  But doing that risks getting milkier, noisier shadows.  There is no free lunch, you pick a black level and contrast level that is the best compromise and you work within that.  You may find that at night, the standard ARRI Rec.709 is too contrasty, you're having to add too much fill light and not capturing enough ambience at night.  Some people will create a lower contrast LUT just for that scenario.  But as I said, there is no free lunch, you are risking getting milkier, noisier shadows. Maybe you can play right on the edge and get what you want, that's great.  The only issue then is flexibility in post, you don't have room later to lighten anything.  But that's not necessarily bad as long as you are consistent and accurate. I don't believe in just giving yourself maximum ability in post to change your mind.

I had a night scene in Paris where I had to use the Alexa at 1600 ISO because the brightness of the Notre Dame in the background was set and I had to use a T2 zoom for the shot.  So I knew I was on the edge of exposure and didn't have the ability to brighten the image in post later unless I resorted to noise reduction software.  But it was OK in the end.

 

maisel_notredame2.jpg

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Thanks so much for the through  explanation. I guess my only other question is there another stock low-contrast LUT or its something I would have to create myself?

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A while back I was shooting with a custom, high contrast LUT, that resembled a film print. So I lit like I did with film.

lately, I’ve been using the built in Rec709 LUT, which is a bit “low contrast”.  And I’m now lighting with rather heigh contrast and often zero fill light, except what bounces around the room.

i wouldn’t worry about the Log recording much as  it actually applies more bits to the shadows than the highlights.

i think my last project was shot close to ISO 1600, but monitored at 800. The set looked really dark, but the recording looks quite nice.

 

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I actually find the standard ARRI Rec.709 LUT to be a bit high in contrast unless they changed it from 10 years ago.  There is (or was) an ARRI LCC (Low Contrast Curve) Rec.709 for recording in the Alexa, designed for people who can't deal with log in post but want more detail recorded than with standard Rec.709... but I don't know if it can be selected as a monitor output LUT only, and then you'd also have to get the post house or whoever is doing dailies to get ahold of the same LCC LUT.  Personally, I'd be wary of monitoring with a low-contrast LUT because you might be tempted to never use fill light.  I'd use a standard contrast LUT, for me it's better to worry about shadow detail rather than assume it's all good.

 

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I think when I say it’s “low contrast” , I’m comparing this Alexa monitor LUT to standard Rec709 video, or even an optical film print.

i understand that compared to how you light and grade your work David, that it can seem to you “higher contrast” 🙂

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